Many kinds of medical suture needles have been used in accordance with their usage, for example, dull needles, square needles, and round needles have representatively been utilized.
A dull needle is used for suturing lever and the like, and a tip of the needle is dull. A square needle is generally used for suturing hard tissue such as skin and muscle, and has a sharp tip and a tapered portion with a shape of a polygonal pyramid, and each prescribed edge on the tapered portion is used as a cutting edge to cut and open a tissue.
A round needle is provided with a body with an appropriate cross-section and a conically tapered portion on the side of a tip thereof. Although the shape of the cross-sections of the conically tapered portion and the body are usually round, in place of the round cross-section, an oval which is sandwiched between two substantially parallel planes, a cross-section enclosed by four planes, and a cross-section which is sandwiched between two substantially parallel planes and of which central portion is constricted may be adopted. However, the tapered portion of a round needle is not adapted to be used as a cutting edge which is obtained by sharpening an edge like a square needle.
After piercing a tissue with a tip, a round needle enlarges the pierced hole with the tapered portion. Unlike a square needle, a round needle is not provided with a cutting edge at the tapered portion, so that the needle does not cut a tissue unnecessarily. Therefore, a tissue at the hole closely contacts the surface of a thread, which prevents body fluid and the like from being spilt from the sutured portion. With this characteristics, a round needle is mainly used for suturing blood vessels and soft tissues.
The above-mentioned round needle is manufactured in the following manner.
At first, a linear (straight shaped) material with a prescribed diameter is cut in a predetermined length. Then, a portion engaging a suture thread is formed at an end of the linear material. As the engaging portion, for instance, a spring eye, or a blind hole which is drilled in an axial direction of the linear material is adopted. Next, a tip and an intermediate portion of the material are ground with a whetstone or the like to form a sharp tip and a tapered portion. Then, rough buff polishing and another polishing with fine whetstone or the like are performed. Then, grinding stripes are removed through buff polishing, electrolytic polishing or the like to make mirror-finished surface. The straight material is bent to form a prescribed shape, and heat treatment and surface treatment are applied to complete the round needle.
In this connection, a demand for the sharpness of a needle, that is, the reduction of impalement resistance has been becoming considerably strong. However, it is difficult to increase the cutting quality of a round needle without cutting edge. In case of a square needle such as a triangular suture needle, as described above, edges at the pyramid-shaped tapered portion function as cutting edges to open a tissue while the needle proceeds in the tissue, so that it is relatively easy to reduce impalement resistance. However, in such a case, the square needle opens tissue with the edges thereof, therefor, cut section becomes large to cause poor sealing property after an impalement of the needle to decrease, resulting in spilt of body fluid. Therefore, the square needle is not suitable for the suture of blood vessels and the like.
To solve the above-mentioned problems, in prior art, the surface of the needle is mirror-finished to reduce the impalement resistance. That is, as a surface treatment in the above-mentioned manufacturing process, a finishing process through buff polishing, electrolytic polishing, chemical polishing or the like is carried out to form a mirror surface.
In the finishing process through buff polishing, cotton cloth, felt, or the like with fine abrasive grain is rotated and is pressed against the material to be ground to cause the abrasive grain to polish and finish the material, which allows a tip and a body of a needle to be mirror-finished.
In the finishing process through electrolytic polishing, electricity is forced to be applied to a needle to melt the surface of the needle through electrolysis.
In the finishing process through chemical grinding, unlike the finishing process through electrolytic grinding, electricity is not forced to be applied, but acid causes the surface of a needle to be melt. In this case also, polish and finished surface becomes mirror surface.
With the above-mentioned finishing processes, although the finished surface looks like a smooth mirror surface with naked eyes, it is not a mirror surface microscopically. For instance, with the buff finishing, many rough stripes caused by abrasive grain are observed.
Further, with respect to the electrolytic grinding, microscopically, it is confirmed that gases generated at the electrolysis adhere to the surface to form a rough surface with shallow craters.
With the chemical grinding, besides the rough surface with craters caused by the gases like the electrolytic polishing, further shallow roughness is formed on the surface of a needle by easily ground crystal grains and hardly ground crystal grains of the material of the needle.
As described above, there are all kinds of mirror-finishing, and whether a complete mirror surface or not is not automatically determined, in other words, the degree of the mirror finishing varies with material to be ground. For example, mirror-finishing for silicon wafer and that for plate member for constructing buildings are different from each other. In the detailed description of the invention, a mirror surface is defined to be such a finished surface of a needle as finished by generally used buff polishing, electrolytic polishing, or chemical polishing.
However, even with the more microscopically smooth mirror surface of a needle, the impalement resistance of the needle is not be reduced. In other words, there is no difference between the mirror surfaces formed through the buff polishing, electrolytic polishing, chemical polishing, and more smooth mirror surface.
In another method in the prior art for decreasing the impalement resistance, a tapered portion is coated with silicon. However, in this case, the silicon coating is peeled off after several impalements, thus, the effect of the silicon is significantly decreased.
To solve the above-mentioned problem, in Japanese Patent Publication No. Heisei 5-18576, it is proposed that channels with craters are formed on a tapered portion of a needle through chemical grinding, and silicon is applied onto the tapered portion. With such construction, silicon is sustained in the crater-like concave portions, and even after the needle is repeatedly used, the increase in the impalement resistance may be prevented.
However, in a suture needle in which silicon is sustained in the crater-like concave portions formed by the chemical polishing, as described above, the concave portions are very shallow as generally called as a mirror surface, the silicon is not so effective.
Further, in another Japanese Patent Publication No. Heisei 5-60746, it is disclosed that, in order to reduce the impalement resistance, a tapered portion near the tip of the needle is formed to be long to decrease the taper ratio (that is, the portion is formed to be thin and sharp).
However, when the taper ratio of the needle is decreased, such a needle is effective to a thin tissue but is not effective to a thick tissue as explained below.
Generally, it is known that there are two peaks of impalement resistance of a round needle. The first peak is observed when the tip of the needle enters a tissue, and the second peak is observed when the end of the tapered portion (the thickest portion) enters the tissue. In other words, the moment the tip of the needle just enters the tissue, the first peak is observed, and after that, the resistance is lowered once. Then, the resistance value gradually increases as the tapered portion becomes thicker, and when the end of the tapered portion, that is, when the thickest portion enters the tissue, the second peak is observed.
A sharper tip of a needle causes the first peak to be decreased. In other words, if a tissue is so thin that the tip of the tapered portion penetrates the tissue, the sharper tip is effective. However, when the tissue is thick in comparison to the long tapered portion, not only the long gentle tapered portion but also a short steep tapered portion contacts the tissue from the tip to the end of the needle, so that work loads of both tapered portions are the same (impalement resistance here is calculated by the following formula: impalement resistance=the coefficient of friction.times.pressure.times.distance that the tapered portion contacts the tissue).
As in the foregoing, the change in the taper ratio allows the pressure to be decreased, however, the distance that the tapered portion contacts the tissue becomes longer. Therefore, the sharper tip is not effective. This is applied not only to a round needle but to a needle with cutting edge only at the tip, so-called a cutting tapered needle. That is, the above problem is applicable to all types of needle without cutting edges at the tapered portion where the cross-section of the needle becomes large.